As demonstrated by teachings appearing in U.S. Pat. No. 4,807,481, the vortex shedding phenomena provides a simple and powerful basis for measuring volume flow rate as well as mass flow rate, as the frequency of vortex shedding is proportional to the fluid velocity and the amplitude of the lift force experienced by a planar vortex detector disposed downstream of the vortex generating bluff body is proportional to the dynamic pressure of the fluid flow that is equal to one half of the fluid density times the square of the fluid velocity. Of course, the fluid density can be determined as a ratio of the mass flow rate to the volume flow rate. It is generally true that the quality and value of a flowmeter is judged by the sensitivity of the flowmeter, which is the ability to measure low fluid velocities. The ratio of the maximum velocity to the minimum velocity measurable by a flowmeter is called the "turn-down" ratio. The best flowmeter is one that has a high turn-down ratio. Many existing vortex flowmeters have turn-down ratios varying in the range bounded by 10 to 1 and 20 to 1 and measure the volume flow rate only. The vortex flowmeter constructed in accordance with the principles taught by U.S. Pat. No. 4,807,481 has a turn-down ratio in the range bounded by 50 to 1 and 100 to 1 and measures volume flow rate as well as mass flowrate. In carefully set-up laboratory experiments the vortex shedding phenomena takes place in a clear and regular manner in a range of fluid velocities equivalent to a turn-down ratio of 1,000 to 1. Consequently, a perfect vortex flowmeter should have a turn-down ratio at least equal to a few hundred to one. In actual practices of the vortex flowmeter, the vibration noises generated by the pipe line vibration and disturbance in the fluid flow interfere with the vortex signals and prevents the detection of weak vortices occurring in low velocity fluid flows. Therefore, the most direct road to a higher turn-down ratio of the vortex flowmeter is to employ a noise sensing detector in addition to the vortex sensing detector, wherein signals from the two detectors are combined in such a way that noise is canceled therebetween and a pure vortex signal is obtained. It is imperative to understand that, in order to cancel the noise signals detected by the noise sensing detector and the noise signals detected by the vortex sensing detector with one another, the two noises respectively detected by the noise and the vortex sensing detectors must be in common node. Otherwise, the task of canceling noise becomes hopelessly complicated and virtually impossible. In order to detect the noise in common node, the arrangement of the noise and vortex sensing detectors must satisfy the following three conditions:
1. The vortex and noise sensing detectors must be disposed at the same cross section of the flow passage. The noise generate by the structural vibrations of the pipe structures and the flowmeter body are generated by the traveling waves and the standing waves of structural vibration. If the vortex and noise sensing detectors are disposed on two different cross sections of the flow passage, the noise generated by the traveling waves propagating through the pipe line or fluid contained therein register noise of two different modes to the two detectors and consequently, they cannot be canceled therebetween.
2. The vortex and noise sensing detectors must be disposed in the interior region of the conduit providing the flow passage. The noise is generated by the vibration of the solid constituting the pipe line and the flowmeter and by the vibration of the fluid medium in the flow passage, which noise is also transmitted through the solid constituting the pipe line and the flowmeter body and through the fluid medium in the flow passage. If the vortex sensing detector is surrounded by the fluid as it is disposed within the flow passage and the noise sensing detector is surrounded by ambient air when it is disposed without the flow passage, the two detectors detect noise transmitted through two different media of different structural arrangements and consequently, having different modes. As a consequence, the noise so detected lack common modes and cannot be canceled.
3. The vortex and noise sensing detectors must have common structural elements, whereby the two detectors have substantially the same natural frequency in the structural vibrations thereof, which is the most serious source of the noise generated by structural vibration of the flowmeter assembly. When the vortex sensing detector is submerged in the fluid, while the noise sensing detector is disposed within an enclosure of heavy construction, the natural vibration of the enclosure housing the noise detecting sensor generates a high intensity noise that is exclusively registered to the noise sensing detector only, and consequently, the vortex and noise sensing detectors detect noise of different modes which cannot be canceled therebetween.